I h i M t f th VLAIonospheric Measurements from the VLA Low-Frequency Sky Survey (VLSS)

Aaron CohenUS Naval Research LaboratoryUS Naval Research Laboratory

H b R iHuub RottgeringLeiden University

Low Frequency VLA

• 74 MHz receivers on VLA– Installed in 1998– Very limited capabilities– Greatest challenge is ionosphere

Crossed dipole suspended belowp psub-reflector by wires

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74 MHz VLA Field of View• Field of View

– Diffraction limited by 25 VLA di h25 m VLA dish.

– ν = 74 MHz λ = 4m.

– FWHM ~ 11.5 deg.– Detects 5-10 celestial

radio sources in 1 minute.

– Detects > 100 celestial radio sources in 1 hour.

– Celestial radio sources act to “back-light” the ionosphere.

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VLSS: VLA Low-frequency Sky Survey• Survey Goals

– 74 MHz VLA in B-configurationMap entire sky above δ > 30o– Map entire sky above δ > -30o

• Survey Method– 523 separate pointings– 75 minutes per pointing– ~700 hours of 74MHz– ~700 hours of 74MHz

observations over a variety of elevations, times of day, times of year.of year.

– An excellent opportunity to obtain statistical picture of ionospheric structure.

• Further Information– Cohen et al. 2007. AJ 134, 1245

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ionospheric structure. ,– http://lwa.nrl.navy.mil/VLSS

The Ionosphere Distorts Observations• Effect on a single

baseline– A spatially varying

ionosphere causes a relative phase delay ybetween two antennas.

– This is equivalent to the geometric phase delay g p yfrom refraction.

– The radio source appears shifted from its normal position.

– The refraction angle can be related to thecan be related to the spatial gradient of the ionospheric TEC.

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Probing Ionospheric TEC Gradients• What does the

VLA measure?– The 74 MHz VLA field of

view, projected to an assumed F-layer height y gof ~400km has a diameter of about 100km.

– Lines of sight from the gVLA to any source probe a patch of ionosphere about 11km in diameter.

– Each source’s position shift determines the relative TEC gradient in an 11km patch.

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Median Differential Refraction (MDR)

• Differential Refraction– Difference in refraction between two

radio sources measured simultaneously.

– Measures difference in TEC gradients between the directions to the sources.

– Depends on:– Angular separation– Elevation– Time of Dayy– Level of ionospheric spatial

variations– Median Differential Refraction (MDR)

is the expected value for a given set of conditions.

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MDR Elevation Dependence• Median Differential

Refraction (MDR)– Increases with greater

source pair separation.– Increases withIncreases with

decreasing elevation.

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MDR Elevation Dependence• Correcting for

Elevation– Assumed a “thin-shell”

model of ionosphere at a height of 400 km.g

– Two major corrections:– Converted angular

distance to spatial separations of pierce points.

– Corrected for path l th th hlength through ionosphere (incidence angle).

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The Ionosphere in Day versus Night• Time Dependence

– Separated the data into d d i htday and night observations.

– MDR much greater d i th dduring the day.

– Fitted power-law exponent is also higher ( t j t l f t )(not just scale factor).

– Conclusion:– Large scale (>100 km)

t t i i ilstructure is primarily responsible for increased MDR during the day.the day.

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MDR Variation throughout the Day• MDR time

dependence– Generally higher during

the day.– Possible “dip” nearPossible dip near

sunrise and sunset.– Fractional diurnal

variations greater forvariations greater for longer spacings than shorter spacings.

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Conclusions• Results:

– Low-frequency radio interferometers can measure fine-scale ionospherict t t hi hstructure at high accuracy.

– “Thin-screen” model of ionosphere appears successful at describing elevation dependencies.

– MDR have been quantified as a function of pierce-point separation and time of day.

– Spatial variations appear greater during the day than at night.– Excess daytime ionospheric structure is primarily on large spatial scales

(>100 km).

• Future Work:– Use MDR to measure power spectrum of ionospheric spatial structure.– Determine MDR variations with respect to archival space weather.

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